1. Field of the Invention
The present invention relates to a trim-tilt device for a marine propulsion unit such as an outboard motor or inboard/outboard motor.
2. Description of the Related Art
Conventionally, the trim-tilt device for a marine propulsion unit has a cylinder device interposed between a hull and the propulsion unit which is tiltably supported by the hull. By controlling supply and discharge of hydraulic fluid from a hydraulic fluid supply/discharge device to a cylinder device or vice versa, the cylinder device is expanded and contracted to thereby trim and tilt the marine propulsion unit.
As disclosed in U.S. Pat. No. 3,250,240, there is, as a cylinder device for a conventional trim-tilt device for a marine propulsion unit, a housing connected to one of a hull and a marine propulsion unit and which forms a large-diameter trim chamber; a cylinder that is telescopically inserted into the trim chamber of the housing and which forms a small-diameter tilt chamber; a large-diameter trim piston that is fixed to an end portion of the cylinder within the trim chamber of the housing which partitions the trim chamber into a first trim chamber of a cylinder accommodation side and a second trim chamber of an anti cylinder accommodation side; a piston rod that is connected to the other of the hull and the marine propulsion unit and that is telescopically inserted into the tilt chamber of the cylinder; and a small-diameter tilt piston that is fixed to an end of the piston rod within the tilt chamber of the cylinder and that partitions the tilt chamber into a first tilt chamber of a piston rod accommodation side and a second tilt chamber of an anti piston rod accommodation side. In this prior art, after the cylinder of the cylinder device projects from the housing outward in a trimming operation, the piston rod of the cylinder device projects from the cylinder outward in a tilting operation.
However, the conventional technique has the following problems.
(1) Since the cylinder of the cylinder device serves as a member for transmitting a constant propulsion force (axial compression force) between the hull and the marine propulsion unit, it is required for the cylinder to ensure a fixed strength for accommodating forces at its small area. As a result, the cylinder is structured with metal material such as iron or the like. At this time, in order to make the cylinder rustproof against water or the like, it is necessary to employ high grade rustproof material such as stainless steel or the like as the material for the cylinder, or to perform rustproof processing such as coating or the like on the cylinder, resulting in increased cost.
(2) Since the cylinder of the cylinder device repeatedly slides on a seal member of a cylinder guide provided on the housing in the region of the trimming operation, rust inducing scratching of the seal member must be prevented from occurring on an outer surface of the cylinder. For this reason, in view of this point, in order to make the cylinder rustproof against water or the like, it is also necessary to employ high grade rustproof material such as stainless steel or the like as the material for the cylinder, or to perform rustproof processing such as coating or the like on the cylinder, resulting in increased cost.
(3) When a hydraulic fluid supply/discharge device and the cylinder device are integrated together, a tank constituting the hydraulic fluid supply/discharge device is provided so as to exhibit a significant bulge laterally about a cylinder proximal portion of the cylinder device, and therefore there is a limitation in compact sizing of the tilt device.
The above (2) is a problem specific to the trim-tilt device, but the above (1) and (3) are problems which are not specific to the trim-tilt device but common to various ordinary tilt devices.